const fs = require('fs');
//先a，再b
fs.readFile('./files/a.txt', 'utf-8', (err, data) => {
    if (err) return console.log(err);
    console.log(data.length);
    //先b，再c
    fs.readFile('./files/b.txt', 'utf-8', (err, data) => {
        if (err) return console.log(err);
        console.log(data.length);
         //先b，再c
    fs.readFile('./files/c.txt', 'utf-8', (err, data) => {
        if (err) return console.log(err);
        console.log(data.length);
    });
    });
});